The exemplary embodiment concerns an electrophotographic printer to print a recording medium with toner particles of a developer mixture that are applied with the aid of a liquid developer or dry toner mixture. In the following, liquid developer is used as an example of a developer mixture in the explanation of the exemplary embodiment without thereby limiting the invention to this.
In such printers, a charge image generated on a photoconductor is inked with the aid of the liquid developer by means of electrophoresis. The toner image that is created in such a manner is transferred indirectly (via a transfer element) or directly onto a recording medium. The liquid developer has toner particles and carrier fluid in a desired ratio. Mineral oil is advantageously used as a carrier fluid. In order to provide the toner particles with an electrostatic charge, charge control substances can be added to the liquid developer. Further additives can additionally be added in order, for example, to achieve the desired viscosity or a desired drying response of the liquid developer.
Such printers are known, for example from DE 10 2010 015 985 A1, DE 10 2008 048 256 A1 or DE 10 2009 060 334 A1.
A print group of an electrophotographic printer essentially comprises an electrophotography station, a developer station and a transfer station. The core of the electrophotography station is a photoelectric image medium that has on its surface a photoelectric layer (what is known as a photoconductor). For example, the photoconductor is designed as a photoconductor roller that rotates past different elements to generate a print image. The photoconductor roller is initially cleaned of all contaminants. For this, an erasure light is present that erases charges that still remain on the surface of the photoconductor roller. After the erasure light, a cleaning device mechanically cleans off the photoconductor roller in order to remove toner particles (possibly dust particles and remaining carrier fluid) still present on the surface of the photoconductor roller. The photoconductor roller is subsequently charged by a charging device to a predetermined electrostatic charge potential. For this, for example, the charge device has a corotron device that advantageously comprises multiple corotrons. By adjusting the current (called corotron current in the following) that is supplied to the corotron device, the charge potential of the photoconductor roller is controllable. A character generator is arranged after the charging device, which character generator discharges the photoconductor roller via optical radiation depending on the desired print image (called discharge potential in the following). A latent charge image of the print image is thereby created.
The latent charge image of the print image that is generated by the character generator is inked with toner particles by the developer station. For this, for example, the developer station has a rotating developer roller that directs a layer of liquid developer towards the photoconductor roller. A BIAS voltage is applied to the developer roller, wherein a potential develops on its surface. A development gap (called a nip) exists between the rollers, in which an electrical field is generated due to the development voltage (formed by the difference between the potential on the developer roller and the discharge potential on the photoconductor roller) applied at the development gap, due to which electrical field the charged toner particles migrate electrophoretically from the developer roller onto the photoconductor roller at the image points at the photoconductor roller. This toner transfer is defined by the field strength of the electrical field in the developer gap. The field develops between the development point on the photoconductor roller (which development point lies adjacent to the developer roller at the developer gap) and the surface of the developer roller (in the following, the difference between the discharge potential and the photoconductor roller at the development point and the potential on the surface of the developer roller at the developer gap is called the development voltage). No toner passes onto the photoconductor roller in the non-image points because the direction of the electrical field that results from the potential on the developer roller and the charge potential at the development point on the photoconductor roller repels the charged toner particles (in the following, the difference of these potentials is called the contrast voltage). The inked image rotates with the photoconductor roller up to a transfer point in which the inked image is transferred onto a transfer roller. The print image can be transfer-printed from the transfer roller onto the recording medium.
The development voltage and the contrast voltage at the development gap should be kept constant to stabilize the electrophotographic printing process. For this, the charge potential on the photoconductor roller can be regulated by the corotron device by adjusting the corotron current (charge regulation), and the discharge potential on the photoconductor roller can be regulated by adjusting the luminous intensity of the character generator (discharge depth regulation). In particular given speed changes, such a regulation can be necessary. However, for this it is required that the potential at the development point on the photoconductor roller can be fixed or that this potential at the development point on the photoconductor roller can be determined.
In order to be able to measure this potential on the photoconductor roller, a potential measurement probe can be arranged between the character generator and the developer station at the photoconductor roller. However, the direct use of the measurement value of the potential measurement probe to regulate the corotron current of the charge device or the luminous intensity of the character generator is not possible since the potential on the photoconductor roller still changes between the location of the potential measurement probe and the development point. The darkness decay rate of the photoconductor roller (thus the spontaneous draining of charge on the photoconductor roller without the effect of light) leads to a reduction of the charge potential both between the charge device and the potential measurement probe and between the position of the potential measurement probe and the development point. The measurement value of the potential measurement probe is less suitable for the regulation of the charge device (darkness decay regulation) depending on how different the distances are between the position of the charge device and the potential measurement probe or between the position of the charging device and the development point. The times that elapse upon rotation of the photoconductor roller depend on these. The effect of the darkness decay rate is therefore also dependent on the rotation speed of the photoconductor roller. However, a direct measurement of the potential at the development point by a sensor is not possible for functional or spatial reasons.
The same relationships also apply to the discharge depth regulation. The discharging of the exposed points on the photoconductor roller is likewise dependent on the time that elapses given rotation of the photoconductor roller between the location of the exposure by the character generator and the location of the potential measurement probe or between the location of the exposure and the location of the development point, and therefore on the rotation speed of the photoconductor roller. In addition to this, the darkness decay rate and the discharge speed are dependent on the environment temperature of the print group due to the changing electron mobility in the photoconductor.